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My house has a zoned HVAC system with 2 zones and a single furnace and a single AC unit. The downstairs zone is about 2200 sq ft and the upstairs zone is only 250 sq ft. There is a duct that runs from the supply side of the air handler directly back to the return side of the air handler. The duct has what appears to be a spring-open damper in it. It appears that the damper is always in the open position regardless of which zone is calling for cooling or heat since there is no power hooked up to it. I thought it might be pressure activated so I ran the fan for both zones today but regardless of which zone was controlling the fan the damper stayed open. It is my understanding that this bypass damper would only be used when the smaller zone is in operation. Should this damper be doing something?

There are a few different Zone Systems on the market today , so, we'd really need to know which you have ...and, if you have multiple thermostats that control a respective duct supply damper , etc... Assuming you have a room thermostat controlling a room duct supply damper...the Bypass Damper you described would start to open up IF alot of the room duct supply dampers were throttling closed due to a load load condition ; in this case, the Bypass Damper would allow the airflow to purposely "short-circuit" and dump back into the return air duct. IN this type of Zone System, the Bypass Damper would be normally closed and pressure buildup inside the duct would open it .

If you dont have the type of Zoned System ive just described, then please disregard. But, to answer your question...no, the Bypass Damper should not be continuously open all the time under any condition.

The zoning is setup as follows: There is a Honeywell Chronotherm IV that controls the downstairs zone and another one that controls the upstairs zone. They can both independently call for heating/cooling. They are hooked to a Honeywell EMM-3 Minizone Panel which then runs to the furnace. The Minizone panel controls the supply dampers to direct the conditioned air to the appropriate zone.

Also, after looking at the setup again I can't be 100% positive of the position of the damper (open vs. closed). I assume that it is open since the external indicator/handle is parallel to the duct. For water or pressurized air systems, I am used to this meaning that a valve is open. Is there anyway I can easily verify the position of the damper? A picture of the damper handle position can be seen at:

the position of the handle does not indicate the position of the damper. the big weight on the end of the handle should hold the damper closed until enough force is placed on it to lift it up. the force is controlled static pressure in the duct work

If you want to verify the position of the Bypass Damper, you could simply make a small slit in that flex duct then peek inside at the damper. Then use some aluminum foil tape to seal the slit you made. The flex duct has the foil outer covering on it, then 1/2" of insulation, then a plastic inner covering with wire that spirals around it for rigidity. A sharp knife will do the trick. Your Bypass Damper should be normally closed and open up upon the zone damper closing down.

Upon watching the damper thru several heating cycles and playing around with the damper I see that it is in fact normally closed. How much should it open up? Is the weight on the arm positioned precisely during the setup of the furnace or is it simply put in a set location? The bypass damper only opens slightly (about 10-15 degrees) when the smaller zone is calling for heat.

HOw much it opens up depends on a few factors such as how much air the fan is moving down the particular main zone supply duct, how big the duct is, how long that zone duct is, how many branch supply ducts are off of it, and how far open the zone damper is. So long as your house is well heated in both zone areas and you have sufficient airflow into all rooms, id leave the Bypass Damper where it is. When the installer put in the system, he typically would spend some time making final adjustments of the Bypass Damper by operating the zone system under varying conditions.